A center pillar, which is a vehicle body part forming a part of a vehicle body of a four-wheel vehicle and is a column between a front seat and a back seat, is formed to have a cross section in the form of a hat by the press forming of a sheet metal. Specifically, the center pillar is a press formed article and is composed of: an intermediate top portion which extends in a longitudinal direction which is also a vertical direction; a pair of blade portions which are both bent from the intermediate top portion toward the inside of the vehicle in a direction perpendicular to the longitudinal direction within the surface of the intermediate top portion, that is, on both sides in a front-and-rear direction of the vehicle, and extend in the longitudinal direction, with a direction of the bend being a direction of the width thereof; and flange portions which extend in the front-and-rear direction of the vehicle away from each other, from tips of the blade portions. This center pillar is formed in an elongated shape having a small dimension of the width in the front-and-rear direction of the vehicle to ensure a wide field of vision from the cabin through the window glass of a side door, but required a high strength for measures against a side collision with other vehicles, walls, and so on.
To increase the strength of the center pillar, there are well-known conventional ways such as the provision of a reinforcement such as a reinforcing material at the center pillar, or hardening treatment on the center pillar.
The former way results in an increase in the whole weight of the center pillar and is against the demand for a smaller vehicle weight, and accordingly, the latter way is more preferable. In the latter way, when performing hardening treatment on the center pillar, it is desirable to perform hardening treatment to be able to achieve a demanded strength distribution by providing a high strength at a position where the high strength is required and to provide a low strength at a position where the low strength is sufficient, in order to effectively receive a load acting on the center pillar.
Japanese Patent Laid-Open No. Hei 10-17933 is well known as a prior art method of performing such a hardening treatment. In this prior art method, a hardening treatment is performed on the center pillar across the entire width in the front-and-rear direction of the vehicle by an induction hardening apparatus. By this hardening treatment, the hardness distribution in the vertical direction is made to correspond to a demanded strength distribution. In order to achieve such a hardness distribution, when the hardening treatment is performed while moving the induction hardening apparatus with respect to the center pillar, the moving speed is changed.
In this prior art, the way for achieving the demanded strength distribution is to create in the center pillar a hardness distribution corresponding to the strength distribution. To achieve such a hardness distribution corresponding to the strength distribution requires a high level technology including control technology in consideration of various kinds of conditions such as a material and so on of the center pillar.
It is an object of the present invention to provide a press formed article such as a center pillar or the like whose demanded strength distribution can be easily achieved by hardening treatment, and to provide an induction hardening method and an induction hardening apparatus for use in producing this press formed article.